The invention relates to a deaeration system for deaerating boiler feed water. In the continuous operation of a boiler, particularly an industrial boiler where steam losses in processing operations occur, it is necessary to deaerate the boiler feedwater which is comprised of steam condensate and makeup water. Oxygen in feedwater delivered to a boiler can cause corrosion and tube failure. In addition to oxygen, carbon dioxide, which is corrosive under high temperatures, must be continuously removed as it is not only present in makeup water, but is produced from the breakdown of bicarbonates in the boiler water.
Deaerators are rated as either 0.03 cc/liter or 0.005 cc/liter, the latter being by far the most effective. The rating indicates the maximum quantity of air remaining in the water after treatment. The 0.005 cc/liter units are used for higher pressure boilers usually over 50 p.s.i. since pressure accelerates corrosion.
Deaerators are classified as atmospheric or pressurized depending on whether the deaerator tank is under pressure. Usually a pressurized deaerator is required to maintain the 0.005 cc/liter standard. An atmospheric deaerator which is vented directly to air is less costly, easier to maintain and safer even though operating pressures in a pressurized deaerator are moderately low.
Certain high performance atmospheric deaerators have been devised with a pressurized water circuit for superheated scrubbing water. The deaerator includes a deaerating section and scrubbing section in a divided tank system. The circulating scrubbing water is superheated in a heat exchanger and sprayed into a scrubbing section of the divided tank where the water flashes any remaining non-condensible gases. High performance systems, such as the Spray Flow Deaerator 4.1 of Industrial Steam as described in my U.S. Pat. No. 3,487,611, issued Jan. 29, 1968 are complex units requiring a high volume of cycling such that the flash steam is sufficient to compensate for surges in low temperature makeup water. Unless properly controlled such deaerating condensers are variable in performance. Additionally, to assure proper atmospheric venting of non-condensible gases, a steam loss is required for the purging process. While the prior art system is suitable where variations in makeup water are minimal to moderate, the system is inconsistent for variations often encountered in boiler systems used for processing equipment. In such applications, return condensate will vary in temperature and volume. such variations will induce surges in makeup water. Since enough steam from the flashed water from the heat exchanger is not available at all times to heat the new makeup water as it enters through the internal vent condenser, the water is inadequately deaerated. In extreme cases, the flashed steam will condense more than desired, drawing air through the vent. For this reason atmospheric deaerators, usually have a protective lining to prevent corrosion.
The deaerator unit of this invention solves many of the problems of a deaerator utilizing a deaerating condenser without a heater in an energy efficient manner.